Do you want to run your iTunes visualizer so it takes up the entire screen without sacrificing your FPS rate? Here's what to do:

Turn on screen zoom in the Universal Access prefs.

In the iTunes Visuals menu, select either "small" or
"medium,: whichever you prefer. Do not select "Fill Screen," as it will not work with screen zooming.

Turn on the visualizer.

Press Command-Option and the "+" key to zoom in until the visualizer fills the screen (use the "-" key to zoom back out).

[robg adds: A unique use of the Universal Access prefs, if nothing else. If you've got a slower Mac, this might be worth trying, though you'll see more pixelation, obviously, than when viewing at normal sizes.]

Another nice touch about this method is that if you have a
multiple monitor set up, the visualizer will now spill over into
the other monitors. (if using visualizer in Full Screen mode, it
will only display on the main monitor, secondary monitors will
black out.)